As is well known, thermal printheads are used for forming intended images by selectively providing thermal energy to thermosensitive paper or a thermal-transfer ink ribbon. Generally, thermal printheads are divided mainly into thin film-type thermal printheads and thick film-type thermal printheads depending on methods of forming their heating resistors. As an example, a typical thick film-type thermal printhead will be described below.
FIG. 1 shows a conventionally used thick film-type thermal printhead 1. As will be described later, a thermal printhead according to the present invention has a structure similar to that shown in FIG. 1 except for its characteristic portions.
The thermal printhead 1 shown in FIG. 1 includes a head substrate 11 formed of alumina ceramic and an additional substrate 20 formed of glass-fiber-reinforced epoxy resin. The head substrate 11 is provided with a linear heating resistor 12, a plurality of drive ICs 13, a common electrode 14 and a plurality of individual electrodes 15. The heating resistor 12 extends longitudinally of the head substrate. The drive ICs 13 are arranged in a row extending in the longitudinal direction of the head substrate.
The common electrode 14 is integrally formed with a plurality of comb-teeth like projections 16 extending parallel to each other. Each projection 16 has a free end electrically connected to the heating resistor 12. Each individual electrode 15 is linear and has two free ends. As shown in FIG. 1, the individual electrodes 15 and the plurality of projections 16 are alternately disposed. One free end of each individual electrode 15 is positioned between two adjacent projections 16 of the common electrode 14 to be electrically connected to the heating resistor 12, whereas the other free end is connected, via a conductive wire 19, to an output pad (not shown) of a relevant drive IC 13. With such an arrangement, the heating resistor 12 includes a plurality of regions 18 each defined between two adjacent projections 16. These regions function as heating dots under the control of the drive ICs 13. Specifically, a current is supplied to the region 18 selected by the drive ICs 13 via the adjacent projection 16 and the individual electrode 15. As a result, the selected region is heated up to function as a heating dot.
The additional substrate 20 is formed with a wiring pattern (partially shown) which is connected to input pads (not shown) of the drive ICs 13 via a plurality of conductive wires 19a. The additional substrate 20 is further provided with a connector 17 connected to the wiring pattern. The connector 17 is also connected to a cable (not shown) for transmitting signals supplied from outside. With the above arrangement, the external signals are transmitted via the wiring pattern to the drive ICs 13. The drive ICs 13 will operate based on the thus transmitted signals.
Each of the drive ICs 13 incorporates a shift register which has a predetermined number of bits corresponding to the number of the output pads of the drive IC 13. The drive ICs 13 have their data-out terminals connected in cascade to their data-interminals, so that the shift registers in the respective drive ICs 13 are connected to each other.
The thermal printhead having the above structure operates as follows. In order to perform printing for one line, printing data for the line need be input to the drive ICs 13 in advance. To this end, the printing data for the line are serially fed to the leftmost drive IC 13 shown in FIG. 1 via the data-in terminal. Then, the printing data are successively fed to the shift registers of the respective drive ICs 13 connected in cascade to each other, and retained in them. In accordance with the retained printing data, the output pads of the drive ICs 13 are selectively actuated in synchronism with a strobe signal fed to each drive IC 13. As a result, the heating dots 18 are selectively heated up for performing a predetermined printing operation.
Unfavorably, the thermal printhead 1 having the above arrangement has the following problems. Since the printing data for one line are serially fed to the drive ICs 13, the printing operation for the line cannot be started until the input of the serial data is completed. This means that, in the above thermal printhead, it is impossible to improve the printing speed beyond a certain limit due to the serial data input. Further, when all the heating dots 18 are actuated simultaneously, an increased amount of current will pass through the common electrode 14. Consequently, the voltage drop along the common electrode 14 is intensified, which leads to uneven printing results.
To deal with the above problems, the following measures have conventionally been taken. To begin with, printing data for one line are divided into a predetermined number of pieces, while the drive ICs 13 are also divided into the same number of groups. Then, each piece of the divided printing data is simultaneously fed to a corresponding one of the groups of the drive ICs 13. Compared with the serial input described above, this method is advantageous in that the printing data can be fed to the drive ICs 13 more quickly, so that the printing speed is improved. In addition, by staggering the timing of driving the respective groups of drive ICs 13, the current flowing through the common electrode 14 will be decreased, thereby reducing the voltage drop along the common electrode 14.
However, the above method suffers the following problem. To feed the divided data to the respective groups of drive ICs 13, a special wiring pattern designed for that particular purpose is needed. Therefore, it is necessary to prepare different kinds of wiring patterns, such as a wiring pattern suitable for the use of two-grouped printing data or a wiring pattern suitable for the use of three-grouped printing data, depending on the characteristics of a device in which the thermal printhead 1 is incorporated or on the need of a user. To individually manufacture such thermal printheads having different kinds of wiring patterns requires additional time and trouble, thereby leading to an increase in cost. Further, in order to actuate the drive ICs 13 group by group with time difference, a wiring pattern for supplying strobe signals needs to be additionally designed in accordance with the particular divisional manner.
Moreover, the design of the various wiring patterns mentioned above may need to be altered after they are produced. For instance, a user may wish to use a wiring pattern designed for three-grouped printing data in place of the originally used wiring pattern designed for two-grouped printing data. Conventionally, in such a situation, a thermal printhead incorporating a wiring pattern designed for three-grouped printing data may need to be purchased additionally, which is very inconvenient.